This invention is directed to an improved process for preparing an ester derivative of a dihydric phenol by reacting a dihydric phenol with an acid anhydride, wherein the improvement comprises carrying out the reaction in the presence of a particular adsorbent selected from silica gel, activated charcoal or a molecular sieve. Also, this invention is directed to treating the ester derivative of the dihydric phenol after its formation with said adsorbents.
Polyarylates are polyesters derived from a dihydric phenol, particularly 2,2-bis(4-hydroxyphenyl)propane, also identified as bisphenol-A, and an aromatic dicarboxylic acid, particularly mixtures of terephthalic and isophthalic acids. These polyarylates are high temperature, high performance thermoplastic polymers which have a good combination of thermal and mechanical properties. They also have good processability which allows them to be molded into a variety of articles.
Many processes have been described in the literature for the preparation of polyarylates. One such process is the diacetate process. In the diacetate process, a dihydric phenol is converted to its diester derivative, which is then reacted with an aromatic dicarboxylic acid(s) to form a polyarylate.
The use of a high purity, low color diester derivative of a dihydric phenol monomer is important in making a low color, stable polyarylate polymer. Conventionally, the purification of such monomers included dissolving the monomers in a suitable solvent, usually with an adsorbent such as activated charcoal and re-crystallizing the product from said solvent. However, such a purification process is complex and uneconomical due to the dissolution and crystallization steps and the large amount of solvent involved.
There are extensive literature references which show the chemical reactivity of charcoal and other adsorbents as in Zechmeister et al, J. Am. Chem. Soc., 64, 1922 (1942). Some charcoals are known oxidizing agents at higher temperatures which would increase the color of most polymers or monomers as discussed in E. Ledoux, "Vapor Adsorption, Industrial Applications and Competing Processes", Chemical Pub. Co., Brooklyn, 1945. In addition, physical adsorption decreases with increasing temperature. The use of the adsorbents of this invention to improve the color of diester derivatives of dihydric phenols in the diacetate process for preparing polyarylates is unexpected since the adsorbent is effective in producing a low color product even though the reaction to produce the diester derivatives is conducted at elevated temperatures.
In the present invention an improved process has been found where high quality low color diester derivatives of dihydric phenols are formed. The improvement comprises the use of a particular adsorbent in the process of preparing an ester derivative of a dihydric phenol from a dihydric phenol and an acid anhydride. No dissolution or re-crystallization is required in the present process. The diester derivative product produced by the process of this invention is used to make very low color, stable polyarylates.